The present invention relates generally to cranes of the type disclosed in U.S. Pat. No. 3,657,969, issued Apr. 25, 1972, and assigned to the assignee of the present invention.
Mobile cranes of the type disclosed in this patent generally consist of a vehicle that has a turntable base or support rotatable about a vertical axis on the frame of the vehicle A crane boom is generally pivoted about a horizontal axis on the support and is raised and lowered by fluid rams.
In recent years, it has become customary to increase the versatility of the unit by forming the boom in a plurality of telescoping sections which are extensible and retractable relative to each other. Generally, these sections are of hollow polygonal configuration and are nestable within each other in a retracted position. The extension and retraction of the sections with respect to each other is generally accomplished through the use of fluid rams having cylinders and piston rods respectively connected to adjacent boom sections so that extension and retraction of the ram will cause extension and retraction of the two sections.
Because of the extreme cost of large units of this type, manufacturers are constantly striving to increase the versatility of the machine without increasing the cost thereof. For example, multisection booms for cranes of this type have been designed so that the maximum length of the boom in the fully extended position approaches 100 feet. However, there are certain limits in lengths of the boom sections which cannot be exceeded and still have the unit capable of being transported along major highways.